Plate Motility

If the true plate motions (for the part of plates exposed on the Globe's surface) are on average gravitationally horizontal (with respect to the geoid), then on boilerplate the movement must too be horizontal with respect to the reference ellipsoid (which is defined to marshal with the geoid on average).

From: Treatise on Geophysics , 2007

Plate boundaries and driving mechanisms

Graeme Eagles , in Regional Geology and Tectonics (Second Edition), 2020

Summary

Plate motion is maintained due to the detailed rest of (one) buoyancy forces generated by their thickness variations, by the distribution and size of their subducted parts and by upwelling in the curtain beneath them, with (2) resistance at their various interfaces with other plates and the underlying pall. The pattern of plate motion may dictate or be dictated past the pattern of mantle convection. The torque remainder maintains, and is maintained by, steady plate move and drape convection over long periods. Relative to their shared margins, movements between pairs of plates may be divergent, forming continental rifts and mid-body of water ridges, convergent, giving rising to subduction and standoff zones, or exist parallel, causing the activeness of transform faults. The detailed structure and development of all these features depend on a wide variety of specific parameters that vary strongly co-ordinate to the speed of relative move, crustal and lithospheric rheology, preexisting structure, and limerick, and the composition and structure of the shallow convecting mantle.

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Crustal and Lithosphere Dynamics

P. Wessel , R.D. Müller , in Treatise on Geophysics (2nd Edition), 2015

6.02.iv.six APM Models, Paleomagnetics, and TPW

APM models accept traditionally been determined for a unmarried plate (e.g., Pacific) or a prepare of plates connected by ridges (e.thou., Africa and neighbor plates in the Indo-Atlantic). It is therefore of interest to propagate these predictions into other oceans using the plate circuits mentioned before and so that APM models may be compared. Every bit mentioned earlier, Molnar and Stock (1987) first showed that the hot spots in the Pacific announced to take moved relative to the set of hot spots used to make up one's mind Indo-Atlantic APM; however, Andrews et al. (2006) found that this relative motion is only meaning prior to 68   Ma. There are two important problems that make these comparisons challenging. First, a key trouble area in designing global plate circuits is the connectedness via West Antarctica. Considering the boundary is located in the Ross Sea, the motion betwixt East Antarctica and West Antarctica remains difficult to make up one's mind with precision. Seafloor spreading in the Adare Trough was get-go identified by Cande et al. (2000), but data remain sparse and relatively small changes in the RPM estimates propagate to give relative big changes elsewhere. In particular, continental deformation within the Transantarctic Mountains is difficult to reconstruct accurately, contributing to the total uncertainty. Second, relatively few hot spot chains have adequate (or any at all) sampling for paleomagnetic analysis, making the models incomplete.

A benchmark test for all global APM modeling has been how well an Africa-based APM, after projection via the plate circuit, is able to reconstruct the tendency of the Hawaiian–Emperor hot spot trail in the Pacific. Early on efforts (Cande et al., 1995; Raymond et al., 2000) showed significant mismatch for reconstructions older than ~   40   Ma and were unable to business relationship for the geometry of the HEB. I approach to address this failure was to consider the outcome of moving hot spots (Steinberger, 2000). The latitudinal component of such motions tin can be constrained past paleolatitudes, at least in the example of the Hawaiian plume (Tarduno et al., 2003). Information technology is expected that recently published data from the Louisville concatenation (Koppers et al., 2012) will exist used to constrain its hot spot move as well. Steinberger et al. (2004) found that by using a moving hot spot APM model for Africa, the predictions in the Pacific depended on which plate circuit was used for the reconstruction. Using the Eastward–West Antarctica excursion employed past Cande et al. (1995) again failed to match the HEB geometry, simply if a new circuit that connected Australia to the Pacific via Lord Howe Rise was used (thus bypassing W Antarctica entirely), the fit improved somewhat. This improvement led Doubrovine et al. (2012) to extend the arroyo of O'Neill et al. (2005) and determine a global APM that satisfied the geometry, age progression, and paleolatitude of 5 hot spot chains (Hawaii and Louisville on the Pacific Plate, Réunion and Tristan da Cunha on the African Plate, and the New England chain on the Due north American Plate). Their model championed the Lord Howe Rising excursion and represents a truly global moving hot spot APM model, despite some shortcomings in fitting the v individual hot spot trails. The Lord Howe Rise plate circuit is based on the assumption that there was no subduction east of the Lord Howe Ascension from well-nigh 100 to 50   Ma. Notwithstanding, recent work (Matthews et al., 2012) shows there is petrologic testify for subduction in this region after xc   Ma, and there is clear seismological testify of subducted slab textile in the lower mantle underneath the Lord Howe Rise region that requires agile subduction between the Pacific and the Lord Howe Rise after ~   xc–85   Ma. This argues for using the Eastward–West Antarctica circuit despite its shortcomings.

The availability of paleolatitudes from the Louisville chain means 2 chains on the same plate now have considerable latitudinal constraints for their oldest sections. Combined with the time serial of inter-hot spot distances (Wessel and Kroenke, 2009), in that location are now several types of data constraints available for APM models. Models of hot spot motion must broadly satisfy these contained datasets. In detail, the inter-hot spot distances tin exist computed given dated stone samples from several hot spot chains; such data are much more plentiful than the limited paleolatitude dataset that requires expensive oceanic drilling trek that hopefully sample enough volcanic flow units to average out the paleosecular variation. Being independent datasets, additions of new chronology and paleolatitude measurements will be exceptionally valuable for future APM studies.

As more paleomagnetic data propose changes in the latitudes of hot spots through time, approaches such as that of Doubrovine et al. (2012) volition likely become refined over time. However, many challenges remain. For reconstructions that go back earlier seventy   Ma, severe numerical artifacts tend to develop in mantle backward advection calculations (eastward.g., Conrad and Gurnis, 2003). Doubrovine et al. (2012) thus made the option to limit the backward advection for their plumes to lxx   Ma; for older times (i.e., up to 130   Ma), a constant menses field was causeless defined past the density structure reconstructed at lxx   Ma. This necessary simplification creates additional doubtfulness for the modeled hot spot motions. Further piece of work will conspicuously be needed to refine global plate circuits and their effects on global APM models, as well as improvements to the APM models themselves.

TPW is divers as the wholesale rotation of the Globe relative to its spin centrality. TPW is believed to occur in response to changing mass distributions about the Globe's spin axis (e.thousand., Tsai and Stevenson, 2007) and has been called upon to explicate tectonic and paleomagnetic puzzles in the Earth's past (eastward.1000., Li et al., 2004). The possibility of TWP complicates the decision of global APM. For example, in fitting their global APM model, Doubrovine et al. (2012) constitute show for significant amounts of TPW. In particular, they identified ii virtually equal and antipodal rotations of the Earth relative to its spin axis for the 90–threescore and 60–twoscore   Ma intervals. Other workers have identified episodes of TPW around 100–110   Ma and several instances in the more than distant by (due east.thousand., Besse and Courtillot, 2002; Greff-Lefftz and Besse, 2012; Steinberger and Torsvik, 2008 ). Paleolatitude measurements from oceanic hot spot chains may contain components of TPW; hence, a key challenge of time to come work is to resolve the contributions from feather motions, TWP, and plate motions.

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Basin Structure, Tectonics, and Stratigraphy

A.Yard. Dayal , in Shale Gas, 2017

4.2 Plate Motility

Plate motion is an important phenomenon of our Earth system. The various geological and geophysical activities on land and sea are the issue of plate motion. Plate boundaries can be divided as convergent, divergent, or transform. They are related with compression, extension, and strike-slip faults. Compression of plates is responsible for subduction activity. Mountain edifice is the result of subduction of one plate under some other plate. There is a standoff of continental plates and also continental plate to oceanic plate. In the case of compression of plates, at that place is crustal shortening, and the thickness of the chaff increases. In the instance of divergent activeness of plates, new ocean floors are created and also related with large-scale volcanic activity and formation of new oceans. Convergence of oceanic plates with continental plates also results in mount edifice activity. Motion of these plates is also responsible for major seismic action every bit information technology activates many large fault systems. In fact, the agile seismic zones at present are related with either tectonic action at continental plates or the move of a plate under continental or oceanic plates. In the case of divergent plate move, new, smaller plates are moving apart. In the case of continental rifts, there is crustal thinning and faulting as the crust undergoes extension. Divergent movement of oceanic plates is responsible for the oceanic crustal thinning and formation of mid-bounding main ridge basalt. The movement of plates is responsible for the formation of new continents and oceans in geological history.

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Mantle Dynamics

D. Bercovici , ... Y. Ricard , in Treatise on Geophysics (2nd Edition), 2015

7.07.5.3 Changes in Plate Motion

Plate motions evolve with various timescales. Some are clearly related to mantle convection, such equally those associated with the Wilson cycle ( Wilson, 1966), that is, the periodic formation and breakup of Pangaea, approximately every 500   My. Various hypotheses have been proposed to explicate the dispersal of supercontinents. The conventional view is that supercontinents provide an insulating coating, and thus, with radiogenic heating, they warm the underlying mantle, somewhen inducing a hot upwelling that weakens and breaks upwards the overlying lithosphere (Coltice et al., 2007, 2009; Gurnis, 1988; Lowman and Jarvis, 1996). However, this mechanism requires more radiogenic heating than may actually exist (Korenaga, 2008). In the aforementioned vein, the present-day continents do not propose that they stand up to a higher place hotter than normal drape; indeed, their basal heat flux appears very low (e.yard., Guillou et al., 1994). Nonetheless, this recent ascertainment might not be by and large relevant; in particular, Rolf et al. (2012) showed that while small-scale subcontinental temperature anomalies occur as continents are globe-trotting, they can become significantly larger when continents assemble into a supercontinent. Regardless, whether supercontinent insulation induces sufficient heating to induce dispersal remains a affair of argue (run into Bercovici and Long, 2014; Heron and Lowman, 2011; Lenardic et al., 2005, 2011).

Plate motion changes that have occurred on shorter timescales are even more difficult to understand. The plate-tectonic history recorded in paleomagnetic data and hot spot tracks consists of long stages of quasisteady motions separated by abrupt reorganizations.

Stages of quasisteady move are reasonably well explained in terms of plate forces (slab pull, ridge push, mantle elevate, etc; Forsyth and Uyeda, 1975) or equivalently convective buoyancy from large-scale heterogeneities (e.m., Lithgow-Bertelloni and Richards, 1995; Ricard et al., 1989). Gradual reversals can also be explained by convective motion; for example, the notion of thermal blanketing can be extended to allow for plate reversals in that hot subcontinental mantle anomalies can effectively annihilate slabs and thus cause radical changes in plate move (King et al., 2002; Lowman et al., 2003) (come across Figure 18 ).

Figure xviii. The convection model of Rex et al. (2002) shows that reversals in plate motion can occur when converging flow over a common cold downwelling (a) draws in hot subcontinental mantle (b) that annihilates the downwelling, afterward changing the polarity of the convergent margin to a divergent one (c).

 Adjusted from Male monarch SD, Lowman JP, and Gable CW (2002) Episodic tectonic plate reorganizations driven by mantle convection. Globe and Planetary Science Letters 203(i): 83–91.

Sharp changes in plate motion, nonetheless, are not hands related to convective processes. The almost dramatic plate motion modify is recorded in the Hawaiian–Emperor bend, dated at 47   Ma (Sharp and Clague, 2006; Wessel and Kroenke, 2008); this bend suggests a velocity change of a major plate of ~   45° during a flow no longer than v   My, equally inferred from the sharpness of the curve. Convective plate driving forces, such as due to sinking slabs, cannot change directions much faster than the fourth dimension to lose or erase a thermal bibelot, which is of order several tens of millions of years, based on descent at a typical convective velocity (encounter Section 7.07.4.1.i ). Therefore, changing convective motions in less than ~   five   My is physically implausible. Abrupt changes may exist due to nonconvective sources such equally rapid rheological response and fast adjustments in plate boundary geometries, such every bit due to fracture and rift propagation (e.thou., Hey and Wilson, 1982; Hey et al., 1995). Plate reorganizations due to (a) the anything of a subduction boundary by rapid slab disengagement and/or continental collision (eastward.g., Bercovici et al., 2015), (b) loss of a ridge and/or trench by subduction of a ridge (e.g., Thorkelson, 1996), or (c) the initiation of a new subduction zone (e.k., Hilde et al., 1977) possibly occur on relatively rapid timescales, although the timing of such mechanisms is non well constrained (Richards and Lithgow-Bertelloni, 1996). Offsets in plate age and thickness along already weak oceanic transform boundaries may provide fantabulous sites for the initiation of subduction (e.g., Hall et al., 2003; Stern, 2004; Stern and Bloomer, 1992; Toth and Gurnis, 1998) yielding a possible mechanism for plate motion changes; this, coupled with the notion of transform faults as long-lived weak 'motion guides' (run into Section vii.07.6.1.2 ), emphasizes the importance of transform boundaries in the plate–mantle organisation. The possible mechanism for sharp plate motion changes underscores the need to understand the interactions between the long timescale convective processes and the short timescale effects associated with the lithosphere'south rheological response, for case, by fracture, error sliding, and strain localization (come across Section 7.07.6 ).

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Plate Tectonics☆

R.C. Searle , in Reference Module in Earth Systems and Ecology Sciences, 2015

Plates as Parts of the Mantle Convection Bicycle

Plate motions are ultimately driven by the Earth's heat energy, and they are intimately related to the mantle convection that is driven by this heat. 1 view of plates is that they simply represent the surficial parts of pall convection cells: as hot, ductile curtain rises to the surface, information technology cools and becomes breakable – a plate – and then moves as a rigid cake over the surface before being subducted, gaining temperature and condign ductile again. Recent results from seismic tomography suggest that, around the rim of the Pacific, sheets of cold material descend beneath subduction zones deep into the lower mantle, implying a strong coupling of mantle movement and subducted plates.

Nonetheless, the coupling is not perfect. There are some parts of the mid-ocean ridge (divergent plate boundaries) where information technology seems that the deeper mantle (below the asthenosphere) may exist descending rather than rising. 1 such place is the so-called Australo-Antarctic Discordance south of Australia. Moreover, some plates, such as Africa, are almost entirely surrounded by ridges and have very few subduction zones on their boundaries. In such cases, a rigid coupling of plates to convection cells would imply the unusual scenario of upwelling forth an expanding ring, with a downwelling column inside it. In fact, ane of the advantages of plate tectonics is that information technology allows partial decoupling of plate motions from deeper mantle flow via the ductile asthenosphere.

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Volcanic Reservoirs and Hydrocarbon Accumulations

Caineng Zou , in Anarchistic Petroleum Geology, 2013

iii Tectonic Environment for the Formation of Volcanic Rocks

Plate motion is restricted past deep-seated processes. Volcanic eruption or intrusion is the reflection of upper mantle and deep crust convection on the ground surface or shallow crust. Therefore, it is of import to consider the tectonic background when studying the distribution and features of volcanic rocks. In terms of plate tectonic theory, volcanism is generally adult in environments that are closely related to plate motion, such every bit basin margins and island arcs ( Figure seven-5).

FIGURE 7-5. Sketch map of the plate tectonic environment.

The rift zone, a canyon topography developed along parallel faults, is i of the major tectonic mobile belts on the footing, which is defined equally large extensional tectonic units with deep affection and large extension (Ma, 1982). Because of upwelling hot drapery or magma, the lithosphere became thin during the extensional procedure. The arching procedure took place commencement, forming big dome structures or the trigeminal rifts and many mistake-blocks, along with continental alkaline and weak-alkaline basalt eruptions in large areas. Magma activities in a subduction zone are mainly inside the range of magmatic arc, about 150–300 km away from the trench axis, and are distributed in an arc shape parallel to the trench. The major rock serial include island-arc tholeiite serial, calc-alkaline metal series, and island-arc alkaline series (or high-M shoshonite serial). Mid-bounding main ridge is characterized by the generation of tholeiite and lack of andesite, where the magma is generated in the relatively shallow depth along the mid-ocean with seismic activities, forming alkaline-poor tholeiite magma (Jokat et al., 1992). Igneous rocks of continental craton are related to certain intraplate extensional tectonic environment. Magma activities usually are related to hotspots or the plume of upwelling drape in the surface area without obvious tectonic features. Magma eruptions inside the oceanic basins are by and large represented by volcanic islands and oceanic volcano, with two bones occurrences: (one) volcanic island chain, and (2) isolated volcanic island. Magma activities are poorly adult in passive continental margins. Except for the preserved igneous associations formed during the continental rifts in the early phase and intercontinental rift stage, there are a few magma activities that are related to drape hotspots or mantle plume, forming some intraplate-like (continental plate and intraoceanic plate) igneous associations, which are dominated past intrusive activities of mafic, ultramafic, and vein rock activities, along with some centered or fissured volcanic eruptions.

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Volume 5

Vivien Gornitz , in Encyclopedia of Geology (2nd Edition), 2021

Plate Tectonics

Tectonic plate motions bear upon paleoclimate modify on timescales of x–100  Ma. Geographic reconfiguration of continents and ocean basins produced marked shifts in North-Due south climate zonation, continental topography, and the opening or closing of ocean gateways that can change sea and atmospheric circulation patterns. Plate collisions raise mountains, while body of water floor spreading changes ocean basin volume and sea level. Volcanism is closely tied to tectonism. Information technology influences climate through atmospheric emissions of CO2, CH4, SO2, HtwoSouthward, and other volatiles.

The onset of modern-style plate tectonics dates to between 3.2 and 2.5   Ga, as inferred from changes in igneous-metamorphic rock assemblages, deformationalstyles, and sulfur isotope anomalies in diamond inclusions (Cawood et al., 2018; Smit et al., 2019).

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Due south Atlantic Ocean

Webster Ueipass Mohriak , ... Andres C. Gordon , in Meso-Cenozoic Brazilian Offshore Magmatism, 2022

Potential field datasets and plate reconstructions

Absolute plate motions based on GPS data in by decades betoken an NW move of the South American plate and an NE movement of the African plate. These directions are in understanding with traces of hot spots, particularly in Africa, where Walvis–Tristan da Cunha is characterized by an NE–SW management, like to the Cameroon lineament near the equatorial margin. According to Fairhead and Wilson (2005), the South Atlantic spreading ridge is moving north relative to the drape, and flow lines associated with plate divergence trend approximately E–West, following the transform mistake zones. The resultant vectors of the accented plate motions point NW forth the Brazilian margin and NE along the African margin (Fig. 1.i). These directions are characterized by major lineaments in the Due south Atlantic Ocean, such as the Walvis Ridge in the African plate and several NW lineaments in the South American plate.

The free-air anomaly map suggests 2 major NW–SE lineaments in the Brazilian margin: the Bahia seamounts in the northeastern margin, and the Cruzeiro do Sul lineament in the southeastern margin, extending from the RGR toward the Cabo Frio High at the purlieus between the Campos and Santos basins (Fig. one.1). The balance Bouguer gravity anomaly map of the eastern Brazilian margin (Fig. 1.two) shows that the Cruzeiro do Sul (Southern Cross) Lineament is marked past an NW-trending zone with postbreakup rift structures in the RGR and igneous intrusions from the oceanic crust toward the continental crust in the Cabo Frio High (Mohriak et al., 2008, 2010).

Several igneous plugs are located both onshore and offshore of the southeastern Brazilian margin, following the NW-trending magmatic lineament in the oceanic region. These volcanic edifices are associated with several large seamounts betwixt the RGR and the Cabo Frio High. The Jean Charcot Seamounts (Fig. 1.two) correspond to the largest cluster of igneous structures adjacent to the distal limit in the Santos Bowl. In the Cabo Frio Volcanic Province, between the Campos and Santos basins, several exploratory wells were drilled in volcanic buildups formed during the Late Cretaceous and Paleogene (Mohriak, 2003, 2020).

Some authors have interpreted that the NW-trending onshore lineament separated the Paraná and São Francisco basins, forming one of the largest intracontinental h2o divides in S America (Ribeiro et al., 2018). Other proposed an nearly E–Westward magmatic lineament onshore, evidenced past several plugs forming a belt parallel to the coast, extending from Poços de Caldas in the west to Cabo Frio Island in the east (Sadwowski and Dias Neto, 1981). Because the radiometric age determinations for these plugs go younger eastward, some authors postulated a hot spot track for these alkaline metal intrusions (Thomaz Filho et al., 2000, 2008). Other authors have proposed changes in the hot spot runway owing to variations in plate motion, correlating to igneous activity in the Santos Basin and to volcanic features in the Campos and Espírito Santos basins (Schattner and Michaelovitch de Mahiques, 2020). However, based on refined radiometric ages (Geraldes et al., 2013), some works have questioned these interpretations.

According to some researchers (Coutinho, 2008), the continental breakup in the South Atlantic margin is related to a triple junction system, as indicated past dyke swarms forth the southeastern Brazilian margin. The south arm of the triple junction trends North–Due south and extends from Florianópolis Isle toward the n, crossing the São Paulo and Rio de Janeiro states. Detailed geological maps of the onshore dykes have characterized a conspicuous NNE tendency for the Early Cretaceous dykes along the southeastern margin, specially in the Florianópolis dyke swarm The northern arm corresponds to the coastal dyke swarms that trend NNE in the eastern function of Rio de Janeiro land (Giro et al., 2021). The NW–SE arm corresponds to the Ponta Grossa Arch, which is clearly expressed on magnetic maps (Almeida et al., 2013). The E–W arm of the triple junction corresponds to the Poços de Caldas–Cabo Frio tectono-magmatic lineament (Sadwowski and Dias Neto, 1981), which runs n of the coastline between São Paulo and Rio de Janeiro. Near the São Sebastião Island, the coastline deflects to an East–Westward direction toward the Cabo Frio region (Fig. 1.3). According to Coutinho (2008), the N–S branch continued south toward the Etendeka volcanics in Namibia, where the dykes align with the N–S arm where the plates are restored to their predrift location. The Merluza Graben, which also trends N–S, is located at the offshore region of the southwestern Santos Basin, and its tendency approximately corresponds to the northern continuation of this triple junction, resulting in a failed arm.

The residual Bouguer gravity bibelot (Fig. 1.2) shows the main elements of the southeastern margin. The map indicates that the pre-Aptian hinge line, corresponding to the proximal limit of the rift basins, is associated with a positive gravity anomaly. This gravity bibelot, which some authors aspect to necking of the continental crust (Meisling et al., 2001), is located offshore the Santos and Campos basins, but deflects toward the onshore region near the Abrolhos Volcanic Circuitous. The inflection of the Cretaceous swivel line indicates that the Early Cretaceous lacustrine syn-rift sediments are restricted to the continental platform of the Santos and Campos basins, whereas the Espírito Santo Basin accumulated a narrow trough of syn-rift sediments in the current onshore region. Landward of the Cretaceous swivel line, but postsalt Cenozoic sediments are observed covering prerift basalts and Precambrian basement rocks (Fig. 1.two).

Some authors interpreted the dissonant NS-trending feature at the southern Santos Basin, marked past conspicuous gravity and magnetic anomalies, to correspond to an aborted oceanic spreading center, known as the AR (Fig. 1.2) oceanic propagator (Mohriak, 2001; Mohriak et al., 2008; Dehler et al., 2016). Other authors interpreted the anomaly to correspond to continental crust based on potential field datasets (Karner, 2000), or to exhumed mantle based on analogies with the Iberian margin (Zalán et al., 2011). The mantle exhumation model assumes that the sedimentary basins in SE Brazil are associated with a magma-poor margin, like to the Iberian margin, where peridotite ridges mark the transition from continental to oceanic chaff (Boillot et al., 1980). Still, the seismic estimation of the southern Santos Basin suggests the presence of seaward-dipping reflectors at the transition to the oceanic crust, indicating a magmatic origin for these features (Gladczenko et al., 1997; Mohriak et al., 2008, 2010; Gordon and Mohriak, 2015; McDermott et al., 2018).

Based on analogies with the distal margin of the Red Body of water, Mohriak (2014, 2018) suggested that the oceanic propagator penetrated the table salt sequences in the southern Santos Basin but was aborted by Late Aptian/Early Albian, attributable to shifting of the spreading center to the due east. Plate reconstructions for the Late Aptian salt basin indicate a 5-shaped structure in the southern Santos Bowl (Müller et al., 2016, 2019). This feature has been interpreted as equivalent to the Gulf of Aden oceanic propagator, which is currently advancing toward the continental crust in the Distant region (Mohriak and Leroy, 2013). In the primal Ruddy Sea, embryonic spreading centers are interpreted at the axial trough, where the protuberant ridge was formed in the past ii one thousand thousand years (Ligi et al., 2012). The seismic estimation of the Blood-red Body of water axial trough suggests that table salt masses are flowing toward an abyss that is floored past proto-oceanic volcanic basement (Mitchell et al., 2010, 2017; Mohriak, 2014, 2018; Feldens and Mitchell, 2015). Contrasting with the Red Ocean, the oceanic propagator in the southern Santos Basin is characterized by a protuberant igneous feature that is locally overlain by thousands of meters of Cretaceous to Cenozoic sediments.

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Plate Tectonics

William R. Dickinson , in Encyclopedia of Physical Science and Engineering science (Third Edition), 2003

Two Quantitative Measures of Plate Motility

Directions of plate motility can exist determined from the orientations of transform faults, which lie parallel to the relative movement of side by side plates, and from the "commencement" (initial) motions of seismic waves generated during earthquakes caused by sudden jerky slip of rock masses in contact along plate boundaries. Most earthquakes do occur along plate boundaries, which are delineated equally faithfully by maps of global seismicity every bit if the plate margins had been traced out by some catholic stylus. First motions of earthquakes along divergent plate boundaries reflect extensional deformation of plate edges, those from the subduction zones of convergent plate boundaries reverberate contractional deformation, and those generated along transform faults point the sense of transform slip, whether dextral (right-lateral) or sinistral (left-lateral) with respect to the fault trend.

Rates of relative plate motility are recorded by arrays of linear geomagnetic anomalies, where earth's magnetic field is greater or less than expected, which prevarication parallel to loci of seafloor spreading inside ocean basins. From independent studies of the magnetization retained by lavas exposed by erosion of volcanic fields on country, it is known that the earth's geomagnetic field has reversed repeatedly through geologic time, to impart alternately normal and reversed magnetization to lavas erupted at different times. The lavas preserve a faithful record of the alternating geomagnetic field because they cooled through the temperature at which the imprint of an ambient geomagnetic field is frozen into solid rock, equally remanent (permanent) magnetization, during successive intervals ("chrons") of normal and reversed geomagnetic polarity. Considering the timing of geomagnetic reversals is irregular (episodic), rather than regular (periodic), the blueprint of normal and reversed polarity chrons in geologic time defines a unique pattern. If normal chrons are denoted by black stripes, and reversed chrons by white or blank stripes, the reversal design is geometrically like to the bar codes used for labeling many commercial products (Fig. 4).

FIGURE 4. Diagram illustrating the correlation of geomagnetic polarity chrons spaced in geologic fourth dimension with seafloor geomagnetic anomalies spaced geographically as magnetic "stripes" detected past magnetometers sailed or flown over the sea floor. The correlation allows the rate of germination of new seafloor by plate deviation at a midocean spreading ridge to be determined without ambiguity.

Analysis of geomagnetic anomalies at sea reveals that the geographic spacing of linear anomalies, positive and negative, mimics the spacing in time of past polarity chrons, normal and reversed (Fig. 4). As successive increments of new oceanic lithosphere form by seafloor spreading, the lavas of the seafloor are magnetized with normal or reversed polarity, depending on the nature of the chron during which each segment of new seafloor was created. Ordinarily magnetized seafloor reinforces the strength of the electric current geomagnetic field, to produce positive magnetic anomalies, whereas reversely magnetized seafloor counteracts the strength of the electric current geomagnetic field, to produce negative magnetic anomalies.

Once each specific geomagnetic anomaly is identified as the record of a detail polarity chron, the geographic spacing of the parallel magnetic anomalies tin exist used as a magnetic record recorder documenting the rate of seafloor spreading induced by plate divergence. Arrays of parallel geomagnetic anomalies are displayed as mirror images on opposing flanks of each midocean ridge marking a divergent plate boundary (Fig. four). Each increment of seafloor is split downwards its middle, where it is hottest and weakest along the plate boundary, by continued plate divergence. Because of that characteristic geodynamic behavior, each midocean ridge generates two identical bar codes, ane displayed on each flank, with each recording half the full spreading rate of plate motion. Correlations of geomagnetic anomalies through the various modern ocean basins allow the relative motions of multiple plates to be established with conviction. The geometric similarity of the "bar codes" delineated in space past geomagnetic anomalies at sea and the "bar codes" delineated in time past polarity chrons shows that seafloor spreading normally gain at rates that are nearly compatible for millions of years (Fig. 4).

Linear axes of seafloor spreading are termed spreading "centers" from the key positions of the youngest geomagnetic anomalies in axisymmetric arrays of magnetic "stripes" (Fig. iv), and from the locations of those key anomalies along the crests of midocean ridges as viewed in transverse profile. The persistent linearity of the magnetic anomalies as they motion away from spreading centers indicates that plates of oceanic lithosphere are indeed rigid, non deforming internally every bit they move laterally with respect to 1 another. Patterns of seafloor geomagnetic anomalies indicate, nevertheless, that the development of spreading centers may include discrete shifts in axes of seafloor spreading ("ridge jumps") into positions breaking older oceanic lithosphere, episodic abandonment or initiation of transform linkages between midocean ridge segments, longitudinal propagation and complementary termination of spatially overlapping axes of seafloor spreading, and development of subordinate local "microplates" bounded by subparallel, simultaneously agile spreading systems.

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Regional tectonics and basin formation: the role of potential field studies – an application to the Mesozoic West and Primal African Rift Arrangement

James Derek Fairhead , in Regional Geology and Tectonics (Second Edition), 2020

Offshore plate tectonic links to the Westward and Central Africa Rift System

The relative plate move for the Cardinal Atlantic, as represented by the Kane fracture zone ( Fig. 20.3), is between the North America and Northern Africa plates. The change in curvature of all the fracture zones indicates subtle changes in relative plate move between these plates.

The fracture zones close to the mid-sea ridge axis are devoid of sediment and are major bathymetric troughs. Farther from the ridge, the fracture zones outset to accumulate low-density sediment and their bathymetric depth increases as the mid-ocean ridge progressively cools and subsides abroad from its axis. These two factors event in sedimentation progressively masking the fracture zones towards the continental margin, making the gravity response of the fracture zones more difficult to place. However, some of the fracture zones tin be clearly tracked close to the continental margins fifty-fifty at the scale of the page image. The isochrones, shown in Fig. 20.five as black lines with x   Ma separation, accept been derived from the geomagnetic reversal data (non shown here due to the sparseness of data and since geomagnetic reversals occur on average every 0.5 million yr).

Effigy 20.5. Free-air gravity map for the Central Atlantic Sea. Superimposed on the map are the isochrons later Müller et al. (2008) at x   Ma intervals with the 0   Ma isochron at the mid-body of water ridge. Below the map are two fracture zone profiles (Kane and Ascension) derived from the fracture zone azimuths and isochrones east of the mid-oceanic ridge. The dashed cherry lines are a subjective attempt to track the linear segments of the azimuth-historic period plots representing smooth plate opening about an individual or slowly moving Euler pole. The red dots are ages of the unconformities identified within the basins of the Due west and Key African Rift System.

Inspection of Fig. 20.3 shows the Equatorial fracture zones, that is the St. Paul'southward fracture zone and those to its immediate s, have a strong gravity response forth their entire length suggesting they accept not been passive, but have been active subsequent to their original development (Mascle et al., 1988). Further south in the northern part of the South Atlantic, the Ascent fracture zone (Fig. 20.3) reflects relative movement betwixt the S America–Southern Africa plates (defined in the lower profile shown in Fig. 20.5). Hither, the fracture zones appear to be passive and the amplitude of the gratis-air anomaly decreases eastwards away from the ridge centrality until it is cut by the SW–NE trending Republic of cameroon volcanic seamount concatenation. To the e of the volcanic chain, the fracture zones are difficult to trace due to the Cameroon volcanic line (CVL) acting as a barrier, with sedimentation being mainly restricted to the east. Unlike the Kane fracture zone, the curvature of the Ascension fractures zone is considerably more subtle, indicating the South America–Southern Africa plate pair has not undergone large changes in relative plate motion during its development. This is clearly seen in the time-azimuth profile plot of Fig. xx.five. These findings volition be shown in Evolution of the WCARS section to provide evidence of the close plate tectonic linkage with the tectonic evolution of the WCARS.

Concrete crustal tectonic linkage between the oceanic and continental domains is essential if changes in direction of plate momentum are to be propagated into Africa. Here, gravity and magnetic information play a significant role in the Gulf of Republic of guinea, at identifying the on- and offshore tectonic linkage beneath the Niger Delta. Here, the Concatenation and Charcot fracture zones of the Equatorial set of fracture zones, identified by the white arrows in Fig. 20.6, cutting the continent–ocean boundary below the Niger Delta before becoming part of the Benue Trough rift structures. Since the Niger Delta is located close to and south of the magnetic equator (~10°N), the full magnetic intensity anomalies tend to follow the boundaries of the SW–NE trending fracture zones, rather than imaging the geomagnetic reversal pattern (Fig. 20.6B). The smoothen spectral response of the magnetic information besides indicates that deep oceanic crust existing beneath a major portion of the Niger Delta while farther onshore (NE from arrowhead), the magnetic field is responding to shorter-wavelength anomalies coming from shallow continental volcanic and basement structures.

Effigy 20.half-dozen. (A) The free-air (offshore) and Bouguer (onshore) gravity field over the Niger Delta region of Nigeria. (B) The total magnetic intensity (TMI) field response over the Niger Delta. The white arrows identify the Chain and Charcot fracture zones and the dashed white line the judge position of the continent–body of water boundary.

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